The usual EPROM memory cells comprise a source, a drain, a floating gate and a control gate, the last two coinciding respectively with a first and second level of polysilicon with interposed dielectric oxide.
Their distribution in a memory matrix can be of the conventional type or of the type described in U.S. Pat. No. 4,792,925 issued Dec. 20, 1988 in the name of this applicant.
Such a `written`, i.e. `programmed`, cell calls for a negative charge on the floating gate which, in the detection phase, maintains the cell in a nonconductive state. A `virgin`, i.e. `unprogrammed`, cell has the floating gate discharged and consequently conducting. Detection of the `nonconducting` or `conducting` state, which can be done with an appropriate detection amplifier, then allows determination of whether the cell is programmed or not.
In this connection one of the most common practical problems encountered with EPROM memory cells is represented by the dielectric defects placed between the two levels of polysilicon. A defect in the dielectric tends to bring on progressive reduction of the charge stored in a `written` cell, resulting in causing the cell to become conducting, i.e. in the state representative of the `unprogrammed` condition. A `programmed` cell with defective dielectric appears then and is sensed in the same manner as an `unprogrammed` cell. This clearly leads to an error in the detection phase.
The object of the present invention is now to make an EPROM memory cell which would be substantially free of detection problems caused by defects in the dielectric.
In accordance with the invention said object is achieved by an EPROM memory cell including a source, a drain, a floating gate and a control gate with dielectric oxide placed between said gates characterized in that it consists of two symmetrical half-cells having the drain and control gates in common, the sources physically separated but electrically connected together, and the floating gates physically and electrically separate.
In this manner any defect in a half-cell of a programmed cell can cause that half-cell to become conducting, but not the other one of the same cell, and a detection amplifier capable of judging a cell conductive only if both the half-cells are conducting can prove insensitive to the defective dielectric and hence give a correct detection.
Since the defects are substantially a statistical fact, it is highly improbably that both the half-cells will be defective. The matrix thus proves to have 100% redundancy without the need of having recourse to particular circuits. This is very useful especially for those devices in which a large EPROM must be combined with various other functions, e.g. with microprocessors, STD cells and so forth. The result is an extremely reliable high-output product.
The present invention can be put in practice with a conventional distribution of cells or of the kind illustrated in the abovementioned Italian patent application no. 23479 A/84 or some other type.